The invention relates to a detecting head for use in apparatus for reading optically an information recorded on a record carrier as a track or tracks.
A typical example of the record carrier on which the optically detectable information is recorded as a track or tracks is a disc shaped record generally called a video disc. In this disc shaped record carrier the information is recorded along a number of concentric tracks or a single spiral track. A known apparatus for reading optically the information recorded on the record carrier comprises an optical system for projecting a light spot onto a track of the record carrier and a position detecting means for detecting a relative position of the information track and the spot of light.
FIG. 1 shows an embodiment of such a known apparatus. In this embodiment a video disc 1 is rotated about an axis 2-2' at a constant speed of, for example, 1,800 r.p.m. A laser beam emitted from a laser light source 3 is focussed by means of an auxiliary lens 4, a reflection mirror 5 and a projection lens 6 onto the video disc 1. In this case the laser beam passing through the record carrier 1 is modulated in accordance with the information recorded in the carrier 1. This laser beam with the modulated intensity is detected by a photodetector 7 which produces an electrical output signal representing the information recorded on the carrier 1. It should be noted that the laser beam may be reflected by the record carrier and the reflected beam may be detected by a photodetector, which in this case is arranged on the same side as the lens 6 with respect to the carrier 1. The output signal from the photodetector 7 is supplied to a processor 8 in which the output signal is processed so as to produce the original information signal V.
In order to read the information out of the carrier 1, the light spot focussed by the projection lens 6 onto the carrier 1 should be aligned accurately on the information track to be read. For this purpose in the apparatus the light beam system including the laser light source 3, the auxiliary lens 4, the reflection mirror 5 and the projection lens 6 are moved relative to the video disc 1 by means of a suitable driving means in a direction perpendicular to a scanning or reading direction at a constant speed (shown by an arrow A in FIG. 1). Additionally in order to compensate deviations of the information tracks and the light spot due to a configuration error of the tracks and an eccentricity of the video disc, the reflection mirror 5 is pivotted about an axis perpendicular to a plane of the drawing so as to control the position of the light spot projected on the record carrier 1. To this end the processor 8 produces a tracking signal C which is supplied to a mirror driving device 9 for rotating the mirror 5. However, in order to effect such an alignment, the projection lens 6 has to satisfy the following conditions simultaneously.
(1) The projection lens should have a property which can focus the laser beam into a spot having a given diameter even if the reflection mirror 5 is rotated.
(2) In general, the amount of the eccentricity of the video disc is hardly limited within 0.1 mm. Therefore, the projection lens should be free from aberration over a range of at least 0.1 mm at the carrier surface.
(3) Since a width of the track on the record carrier is normally smaller than 1 .mu.m, the projection lens should have a numerical aperture of about 0.4.
(4) In order to focus the light spot onto the carrier surface, the projection lens must be moved in the direction perpendicular to the carrier surface. Thus, the weight of the projection lens should be light, possibly lighter than 10 grams.
In the case of designing the projection lens which satisfies the above properties, the number of lens components is naturally increased. Therefore, it is difficult to obtain a lens which is sufficiently light in weight. Moreover, the cost for manufacturing such a lens is increased.